1. Field of the Invention
The present invention relates generally to a phase-tracking loop circuit for processing signals whose frequency fluctuates, such as FM signals, PM signals, or AM signals whose carrier frequency is unstable.
2. Description of the Prior Art
Phase-tracking loop (PTL) circuits have been proposed, particularly for use as detectors for FM signals. One such proposal is discussed in Ishigaki et al., Phase-Tracking Loop (PTL) Detector for FM Signals and its Application to an FM Receiver, IEEE Transactions on Consumer Electronics, Vol. CE-24, No. 3, August, 1978, pages 215-225. Such a PTL circuit utilizes a tracking filter, which is a narrow band variable tuning filter, a phase shifting, or Hilbert filter, a phase comparator, a loop filter and a loop amplifier. The phase comparator provides a feedback signal to tune the tracking filter to track the instantaneous phase variation of the incoming FM signal. The phase shifting Hilbert filter associated with the PTL circuit is designed to exhibit a flat amplitude characteristic and to provide a uniform 90.degree. phase shift. To provide such a characteristic, the Hilbert filter normally consists of a differentiator followed by a limiter, although it could also be formed of an integrator followed by a limiter. In either case a flat output is provided, and the phase of the output signal of the Hilbert filter is shifted +90.degree. or -90.degree., respectively, so that a 90.degree. phase-shifted signal can be applied to the phase comparator for comparison with the signal passed by the tracking filter.
Unfortunately, such PTL circuits have been quite susceptible to misoperation caused by interference, noise, or other stray signals above or below the bandwidth of the tracking filter. If a differentiator circuit is used as a phase shifter, any interfering signal having a frequency higher than the signal being processed will be emphasized in the differentiator circuit and fed to the phase comparator. It is apparent that such high frequency interference or noise will be compared with the output of the tracking filter, and will thus prevent accurate phase tracking of the input signal.
Alternatively, if an integrator circuit is used as the phase shifter, any interference or noise signals having a frequency lower than the input signal will be emphasized, and will also prevent accurate phase-tracking of the input signal.
One possible use of the phase-tracking loop circuit, to wit, in providing the index signal for a beam-index color cathode ray tube in a video receiver, is made more difficult because of the above-mentioned susceptibility of the PTL circuit to interference or noise. Several sources of such interference and noise are inherent in a beam index color television receiver, such as variations in the beam scanning velocity, variations in cathode ray or electron beam current, and inherent noise in the photodetector used to provide the index signal. These examples of interference and noise are quite difficult to avoid by design of the television receiver itself, and thus require an improvement in the PTL circuit, such as that offered according to the present invention.